I did some testing with the Porsche over lunch at the same track and I was struggling to get the tyres to overheat on the default setting. I had to turn off the TC and ABS, then keep spinning the rears up to get them into the orange, but by the end of the main straight they were back in the green. I suspect it might have been setup related. Have you ever driven the car before on a different version of the game and made changes to the setup? The reason I ask is because it is recommended to reset the setup to default after an update to the game as people have reported odd behaviour when trying to carry their setups forward.